Mutual Coupling in RIS-Aided Communication: Model Training and Experimental Validation

Mutual coupling is increasingly important in reconfigurable intelligent surface (RIS)-aided communications, particularly when RIS elements are densely integrated in applications such as holographic communications. This paper experimentally investigates the mutual coupling effect among RIS elements using a mutual coupling-aware communication model based on scattering matrices. Utilizing a fabricated 1-bit quasi-passive RIS prototype operating in the mmWave band, we propose a practical model training approach based on a single 3D full-wave simulation of the RIS radiation pattern, which enables the estimation of the scattering matrix among RIS unit cells. The formulated estimation problem is rigorously convex with a limited number of unknowns un-scaling with RIS size. The trained model is validated through both full-wave simulations and experimental measurements on the fabricated RIS prototype. Compared to the conventional communication model that does not account for mutual coupling in RIS, the mutual coupling-aware model incorporating trained scattering parameters demonstrates improved prediction accuracy. Benchmarked against the full-wave simulated RIS radiation pattern, the trained model can reduce prediction error by up to approximately 10.7%. Meanwhile, the S-parameter between the Tx and Rx antennas is measured, validating that the trained model exhibits closer alignment with the experimental measurements. These results affirm the accuracy of the adopted model and the effectiveness of the proposed model training method.

I. Introduction

Standing as a groundbreaking technique in modern wireless communication systems, reconfigurable intelligent surfaces (RISs) represent a revolutionary paradigm for the smart electromagnetics (EM) wave control [1], [2], [3], [4]. These surfaces, composed of an array of programmable reflective or refractive unit cells, empower real-time adjustments to the radio environment. Leveraging advanced metamaterials and signal processing techniques, RIS enables precise modulation of the phase, amplitude, and polarization of EM waves. Such adaptability promises to significantly benefit wireless applications including communication, sensing, localization, and mapping [5], [6], [7], [8], [9]. With the advent of densely integrated RISs such as holographic RIS [10], the phenomenon of mutual coupling emerges as a critical consideration with profound implications for the performance of RIS-assisted communications [1]. Mutual coupling refers to the interaction between adjacent RIS unit cells, wherein the EM fields generated by one unit cell influence the behavior of its neighboring unit cells. This inter-element coupling effect can significantly impact the overall functionality and effectiveness of a RIS deployment, especially in scenarios involving high amplitude gains or varying operational frequencies. Therefore, understanding and mitigating the impact of mutual coupling is essential for optimizing the performance of RIS systems.